Novel monopropellant



United States Patent O sylvania No Drawing. Filed June 13, 1960, Ser. No. 36,986

6 Claims. (Cl. 149-109) This invention relates to monopropellant rocket fuels which are improved monopropellants for auxiliary power generation. In particular this invention deals with novel monopropellants comprising a fuel and an oxidizer, which monopropellant has improved thermal stability.

In rockets and missiles which contain control and navigation equipment, auxiliary power units are often used to provide the energy necessary to activate the various mechanisms. Frequently, these auxiliary power units are also rocket engines which use their exhaust gases to. drive electric generators. Because of the special conditions under which some auxiliary rocket engine power units must function, the monopropellants which they employ must meet very unusual specifications. For example, the monopropellants used in such rocket engines must be thermally stable and capable of withstanding elevated temperatures for extended periods of time prior to satisfactory ignition and performance. In addition,

the monopropellant must have a suitably high specific.

impulse at the chamber pressure during use. Still other requirements for the monopropellants are their ready ignition, low shock sensitivity, non-corrosive properties to the metals used in the engine, and they should form no solid exhaust products which build up on the generator vanes causing imbalance.

Now according to this invention, a monopropellant composition having improved thermal stability and suitable for use in rocket engines for auxiliary power units is provided by the novel composition comprising a mixture of perchloric acid dihydrate as oxidizer and as fuel an alkane or fluoroalkane sulfonic acid containing from one to three carbon atoms. It is particularly surprising that this combination of fuel and oxidizer is thermally stable in view of the fact that most organic compounds, including other fuels such as unsymmetrical dimethylhydrazine and isopropyl nitrate, are decomposed by perchloric acid dihydrate. In fact, this technique of decomposing organic compounds with perchloric acid dihydrate is the basis of an analytical procedure in which another acid is always present in order to modify the decomposition and avoid explosions which occur with the perchloric acid alone.

The techniques by which the monopropellants of this invention are used in auxiliary engines are well known in the art. The liquid monopropellant is held in a storage tank until ready for use. When electrical energy is to be generated from the monopropellant, a valve assembly is automatically opened and the monopropellant is passed through an injector face and sprayed into the rocket motor chamber. The propelling means to carry the propellant into the rocket chamber is preferably gas pressure (e.g., an inert gas such as nitrogen, argon, etc.) under which pressure the monopropellant is held in the storage tank. Less preferably, because of its undesired weight, an electrical pumping system may be used. Ignition of the propellant is readily accomplished with an electric glow plug within the rocket chamber and after the initial ignition, the burning of the monopropellant is self sustaining. As the gaseous products from the combustion exit from the rocket chamber they turn turbine blades of an electrical generator which delivers the desired electrical energy.

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The preparation of the novel monopropellant is carried out simply by adding the oxidizer in small increments to the fuel. The fuel is pro-cooled to below room temperature before the oxidizer addition and it is usually necessary to maintain cooling to hold the temperature below about 35 to 40 C. as the perchloric acid di-hydrate is added. The amounts of fuel and oxidizer used will be in a ratio to supply essentialy the stoichiometric amount of oxygen required to permit complete combustion of the carbon, hydrogen, fluorine, chlorine, and sulfur components of the system. However, the amounts of fuel and oxidizer need not be stoichiometric, although a loss in performance may occur if the oxidizer is much in excess. It is frequently desirable to use a 10% molar excess of fuel in order to obtain a slight improvement in performance. The resultant monopropellant is a clear water-like liquid which is handled and stored in accord with the techniques usually used with high energy materials.

As indicated, the fuel which will be used to prepare the monopropellant of this invention will preferably be selected from the class of alkane sulfonic acids and fiuoroalkane sulfonic acids containing from one to three carbon atoms. The alkane sulfonic acids containing more than three carbon atoms are not stable in the presence of the perchloric acid dihydrate and thus cannot be used. Examples of useful fuels include methanesulfonic acid, ethanesulfon ic acid, n-propanesulfonic acid, trifiuoromethanesulfon-ic acid, B,B,,B trifluoroethanesulfonic acid, and the like. Of this class the preferred fuels are the fluoroalkane sulfonic acids, particularly 5,5,,8-trifiuoroethanesulfonic acid. This preferred fuel is prepared readily from the bis(triiluoroethyl) disulfide disclosed in U.S.P. 2,894,991 by chlorination to the sulfonyl chloride and hydrolysis to the sulfonic acid.

The following examples will further describe the invention. All parts are parts by weight.

EXAMPLE 1 A monopropellant was prepared by adding with agitation 58.62 parts of perchloric acid dihydrate to 41.38 parts of methanesulfonic acid. The resulting solution was a clear, water-like liquid. The amounts of fuel and oxidizer are stoichiometric quantities for complete combustion. The specific impulse for this monopropellant is 150 seconds as calculated for a frozen equilibrium at a pressure ratio of P /P =300/l4.7 where P is chamber pressure and P is exit presure.

EXAMPLE 2 As in Example 1, to 35.03 parts of ethanesulfonic acid there was added with mixing 64.97 parts of perchloric acid dihydrate to obtain a clear, water-like liquid monopropellant.

The specific impulse for this monopropellant is 169 seconds as calculated by the method of Example 1.

EXAMPLE 3 Chlorine gas was passed through a stirred mixture of bis(trifluoroethyl) disulfide (30 g.) in ml. of Water at 6070 C. for two hours, after which time chlorine was no longer absorbed. After standing, the layer heavier than water was separated and dried over anhydrous magnesium sulfate. Vacuum distillation gave 32.5 g. (70% yield) of the colorless liquid, trifluoroethanesulfonyl chloride, B.P. 65 C. (45 min); 14:, 1.3873.

Analysis.Calcd. for C H CIF O S: C, 13.16; H, 1.11; Cl, 19.40; S, 17.57. Found: C, 13.40; H, 1.12; Cl, 20.02; S, 17.27.

Hydrolysis of 20 g. of trifluoroethanesulfonyl chloride by refluxing with 50 ml. of water for three hours gave a quantitative yield (18 g.) of trifiuoroethanesulfonic acid after removal of the water under vacuum. Vacuum distillation of the acid gave the colorless, hygroscopic solid (CF CH SO H); Bl. 100 C. (0.5 mm.); MP. 50- 52 C.

As in Example 1, 61.58 parts of Bfifi-trifiuoroethanesulfonic acid and 38.42 parts of perchloric acid dihydrate were mixed to obtain a Water-like liquid monopropellant.

The specific impulse for this monopropellant is 125 seconds as calculated by the method of Example 1.

Monopropeliant Evaluation The above monopropellants were evaluated for thermal stability, ease of ignition, shock sensitivity, and com patibility with metals. The following description gives test methods and results obtained:

Thermal Stability Weighed samples of each of the monopropellants were heated to 249 C. and held at this temperature for eight hours. After cooling, the samples were reweighed and the percent of weight loss determined. Table I illustrates the results obtained:

TABLE I Percent Monopropellant held at 249 C. for 8 hours: weight loss Monopropellant of Example 1 15.0 Monopropellant of Example 2 14.2 Monopropellant of Example 3 3.4

The above weight losses are indicative of only minor decomposition and the monopropellants are considered to have adequate thermal stability for use at the test temperature. Previously known monopropellants (e.g., dimethyltriethylenediammonium dinitrate in nitric acid) have a maximum permissible exposure below 121 C. and thus cannot be used in systems where temperatures above this limit are reached.

Ease of Ignition Shock Sensitivity A glass vial containing the monopropellant of Example 2 was dropped from a fifty feet height onto a concrete 4 surface without exploding. In general, other propellants now in use (e.g. hydrazine based propellants) will explode under this shock test.

With the monopropellant of Example 1, a piston impact from a two kilogram weight falling twenty-four inches was required to cause explosion. A propellant requiring this degree of shock impact to explode may be safely handled.

Compatibility The monopropellants of Examples 1, 2, and 3 did not attack copper or aluminum and may thus be contained in and transported through equipment of these materials.

It is evident from the above experimental and calculated data for the monopropellants of this invention that they are useful in rocket engines for auxiliary power units. Their particular value is their surprisingly high th rmal stability which permits their use in rocket systerns where the temperatures reached are as high as about 250 C. Because they can withstand such temperatures the auxiliary engines in which they are used will function dependably and not malfunction because of propellant decomposition.

It will be understood that many variations may be made from the above description and examples without departing from the spirit and scope of the invention.

We claim:

1. A novel monopropellant consisting essentially of perchloric acid dihydrate as oxidizer and a fuel selected from the group consisting of alkane and fluoroalkane,

sulfonic acids containing from one to two carbon atoms, said oxidizer and fuel being in amounts to provide essentially stoichiometric quantities for complete combustion.

2. The monopropellant of claim 1 wherein the fuel is an alkane sulfonic acid.

3. The monopropellant of claim 1 wherein the fuel is methanesulfonic acid.

4. The monopropellant of claim 1 wherein the fuel is ethanesulfonic acid.

5. The monopropellant of claim 1 wherein the fuel is a fiuorosulfonic acid.

6. The monopropellant of claim 1 wherein the fuel is fi,fi,,8-trifiuoroethanesulfonic acid.

References Cited in the file of this patent UNITED STATES PATENTS Pino June 5, 1956 Barr et a1. July 14, 1959 OTHER REFERENCES 

1. A NOVEL MONOPROPELLANT CONSISTING ESSENTIALLY OF PERCHLORIC ACID DIHYDRATE AS OXIDIZER AND A FUEL SELECTED FROM THE GROUP CONSISTING OF ALKANE AND FLUOROALKANE SULFONIC ACIDS CONTAINING FROM ONE TO TWO CARBON ATOMS, SAID OXIDIZER AND FUEL BEING IN AMOUNTS TO PROVIDE ESSENTIALLY STOICHIOMETRIC QUANTITIES FOR COMPLETE COMBUSTION. 